Study finds southern rivers ripe for clean-living didymo

JON BRAY: The impact of didymo is greatest in some of our most pristine waterways.

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Didymo, or rock snot, has been put under the microscope in one of the largest field studies of its kind.

Undertaken by the University of Canterbury, the project sampled 55 rivers and streams across the South Island and the highest biomass site sampled was the Waitaki River.

Didymo, originally a species of algae native to the northern hemisphere, is rapidly spreading in many temperate regions and forms large cotton wool like masses smothering river and stream bottoms.

However, its effects on ecosystems is only noted where it blooms.

These habitats are low nutrient, dam and lake-fed rivers where lakes and dams reduce the effect of floods, which removes didymo.

One of the university's freshwater ecology PhD student researchers, Jon Bray, said a number of other regulated rivers frequently suffered the effects of severe didymo blooms.

"These included the South branch of the Ashburton river, and the Ohau and Tekapo amongst others. Unfortunately the often low nutrient waters of our inland rivers, coupled with the stabilising effects of dams, provide the right conditions for bloom formation," Bray said.

A number of South Canterbury rivers provided the right conditions, but there were high biomass sites all over the country.

"Blooms usually form during summer where high light and nutrient deprivation causes growth of the stalks didymo uses to attach to the stream bottom."

Bray said their investigations found that didymo presented a catch 22 scenario.

"The didymo spread seems to have slowed substantially in recent years so for the moment the situation is not getting better or worse."

Didymo has not invaded the North Island despite the presence of suitable habitat.

"Didymo is a lover of clean water and it blooms in response to low levels of water nutrients which is the opposite of what we may expect. Thus its impact is greatest in some of our lowest nutrient or pristine waterways."

Didymo is absent from waterways that are suffering the effects of agricultural practices.

"Invasive organisms, such as didymo, are recognised as a major threat to biodiversity globally.

"Globally, freshwater ecosystems are now considered some of the most impacted from the effects of invasives."

Bray said while tools to eradicate and manage didymo have been investigated, prevention of spread is regarded as the only viable long- term policy.